Cerium washcoat

In addition to platinum and related metals, the principal active component ia the multiflmctioaal systems is cerium oxide. Each catalytic coaverter coataias 50—100 g of finely divided ceria dispersed within the washcoat. Elucidatioa of the detailed behavior of cerium is difficult and compHcated by the presence of other additives, eg, lanthanum oxide, that perform related functions. Ceria acts as a stabilizer for the high surface area alumina, as a promoter of the water gas shift reaction, as an oxygen storage component, and as an enhancer of the NO reduction capability of rhodium. 

A washcoat, which provides a high surface area onto which the active catalyst is impregnated. The washcoat typically consists of a mixture of zirconium, cerium and aluminium oxides. Apart from providing high surface area the washcoat also acts as an oxygen storage system (see below). 

It was noted in the introduction that the washcoat of the automobile catalysts contains several other oxides, mainly cerium and lanthanum oxide. 

The chemical composition of the washcoat belongs to the core know-how of the catalyst manufacturers. The most common washcoats contain aluminum oxides, cerium oxides and zirconium oxides as major constituents. The minor constituents... 

Nowadays, some of the washcoat internal surface area is provided by cerium and zirconium oxides. Those oxides are available in a modification that exhibits a moderately high internal surface area, typically 20-100m g , together with an appreciable stability of this internal surface area at typical catalyst operating temperatures. 

The main task of the cerium oxide washcoat component is oxygen storage, because the cerium ion is easily reduced and oxidized under typical catalyst operating conditions, formally according to the reaction... 

Zirconium oxides are the preferred supports for the precious metal component rhodium. The cerium oxide and/or the zirconium oxide are added to the washcoat either as preformed oxides or as oxide precursors, such as their respective carbonates or nitrates - the oxides are then formed in situ during washcoat drying and calcination. 

Similarly, Fig. 75 shows the effect of both the amount and the type of precious metal on the extent of the solid state reaction between aluminum oxide and cerium oxide, leading to the formation of cerium aluminate. The overall effect of these interactions is that the catalytic activity and the durability of the precious metals vary as a function of the type of washcoat oxide they are deposited on [45]. 

Each catalyst was studied by XRD. To improve the intensity of the diffraction lines, the washcoat was scraped off the cordierite before analysis. The XRD spectra show the presence of alumina, ceria and some residual cordierite. The precious metals are never detected, as metal or oxide. For the firesh catalysts, the lines are broad meaning poorly crystallized phases. The resolution is greatly improved after aging. No other phase like cerium aluminate or other transformation product were evidenced on the XRD spectra. 

Within the current TWC catalyst washcoats, rhodium is susceptible to deleterious interactions with various components during a prolonged lean high temperature excursion. To elucidate the potentially detrimental rhodium compounds formed under such circumstances, unsupported rhodium oxides, rare earth metal rhodates, and aluminum rhodate are characterized and measured for catalytic activity. The intrinsic activities at 673K of NO, CO and CjHg conversions over various unsupported rhodium oxides species are basically structure insensitive. However, the intrinsic activities at the same temperature of both the rare earth metal rhodates and aluminum rhodate appear to be sensitive to their structure. The interaction between rhodium and the rare earths especially cerium, is found to be much stronger than that between rhodium and aluminum. 

TWC used in the present study are commercially available monoliths, from the same manufacturer, with around 20 weight % of washcoat (3.6 weight % of cerium) loaded with 40g/ft3 of precious metals (PM), with a Pt/Pd/Rh ratio of 5/0/1. Three TWC were studied by the various analytic methods ( sections 2.2 and 2.3) a fresh catalyst (42 m /g), a 130 000 km vehicle aged, and a catalyst treated 24 h in air at 1100°C (3 m /g). Two samples were picked from the 130 000 km catalyst at the inlet (17 m /g) and at the outlet section (15 m /g). Four solids were studied only by O2 TPD and H2 TPR a washcoated cordierite without any PM, a TWC treated 4h under N2 at 1050°C, a commercially available catalyst 0.5% Pt/Al203 and a sample of pure Ce02 (4.4 m /g). 

In order to identify the cerium oxide contribution on the H2 TPR spectrum of the washcoat (Fig. 4), a TPR have been carried out on pure Ce02 oxide. The spectrum obtained (Fig. 5), indicates two reduction processes. The first one appears, as a well defined H2 consumption peak at Tm= 604°C (169 pmolH/g). The second process is noticed over a wide range of temperatures (T> 650°C) and is incomplete at 900°C. 

The TPR spectrum of pure Ce02 oxide (Fig. 5) is similar to those noticed on cerium oxides of various specific surface areas [3,10]. The first peak (T = 604°C) may correspond to the reduction of a surface oxygen species related to Ce [3]. Johnson et al [10] have shown that the position of this peak shifts to lower values of Tm when the specific surface area of the solids increases. The hydrogen consumption detected at high temperatures (Fig. 5) was also noticed by the various authors [3, 10] and was attributed to the reduction of the bulk Ce02 oxide. The differences in the TPR spectra of the washcoat without PM and of the CeOo oxide can be interpreted by the existence of interactions between CeO and AI2O3 [3]. 

The behaviour of a Pt-based catalyst on a metallic monolith support washcoated with alumina, with the addition of lanthanum and cerium, was studied by Musialik-Piotrowska and Mendyka. The activity of the catalyst was tested in the oxidation of ChB and DCE alone and in two-component mixtures with toluene, -hexane, acetone, ethanol, and ethyl acetate. The influence of non-chlorinated compounds on ChB oxidation differed from one compound to another. Over the whole range of reaction temperatures, ethanol enhanced the conversion of ChB by 10%. The addition of both hydrocarbons also slightly improved ChB destruction, while DCE conversion was inhibited in the presence of each non-chlorinated compound that was added. Both chlorinated hydrocarbons not only inhibited catalytic destruction of each of the non-chlorinated compounds added, but also increased the reaction selechvity and concentration of the intermediate yielded, the first of which was acetaldehyde. 

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